Soybean protein powders have been used in a wide variety of foods such as processed meat products, fish paste products, frozen cooked foods, beverages, desserts and the like for various purposes, for example, to improve physical properties (e.g., gel formation ability) of foods, to enhance nutrition of foods, to reduce manufacturing cost of foods and the like. However, a soybean protein powder has dustability and, when it is taken out from a package, it shows so-called "dusting". Further, when a soybean protein powder is simply admixed with water by stirring, it forms undissolved lumps, which makes it difficult to prepare a solution or a paste of soybean protein powder within a short period of time.
Hitherto some methods have been proposed for improving properties of a soybean protein powder such as water dispersibility and the like. For example, Japanese Patent Publication Nos. 6817/1971 and 52542/1981 disclose that a soybean protein powder having improved properties can be produced by heating an aqueous solution of soybean protein for a certain period of time with a jet cooker and then spray-drying. Further, Japanese Laid Open Publication No. 35449/1976 discloses that a soybean protein powder having improved properties can be obtained by adding a surfactant such as lecithin to a solution containing soybean protein, homogenizing the resulting mixture and then spray-drying. However, these conventional methods provide less satisfactory improvements.
Further, in the production of a milk powder, such a technique that a starting powder material is agglomerated by using a so-called instantizer to increase its particle size by 30 to 150 times as large as its original size has been commonly employed to continuously produce the milk powder in a large scale. However, this technique is hardly applicable to the production of a soybean protein powder. That is, in the case of a milk powder, particles tend to be immediately agglomerated with steam to readily form larger particles. In addition, coating can be readily performed, because the starting powder material scarcely contains fine particles which can pass through a 200 mesh screen (number of openings/inch) and, probably, lactose contained in the powder material functions in this agglomeration. In contrast to this, a soybean protein powder contains a substantial amount of fine particles which can pass through a 200 mesh screen and soybean protein is more liable to undergo heat denaturation than milk protein. Therefore, it is very difficult to agglomerate soybean protein by using an instantizer while preventing the deterioration of the physical properties such as gel formation ability. Furthermore, even in the production of a milk power, it is said that agglomeration of particles with a fluidized bed in a continuous process is very difficult [Syokuhin Kogyo, Vol. 24, No. 16, p 46 (1981)].
In order to reduce dustability and to improve water dispersibility of a soybean protein powder, the present inventors have intensively studied agglomeration of soybean protein powder particles and the use of surfactants. As the result, it has been found that, apart from the production of an agglomerated product having a particle size of more than two or three hundreds microns like a conventional agglomerated product, some advantages can be obtained by using a fluidized bed. That is, it has been found that, when a soybean protein powder is treated in a fluidized bed, it is possible to reduce its dustability and to improve its water dispersibility and other properties, even if the particle size thereof is about one hundred and several tens microns or less. It has been further found that it is possible to perform the treatment in a fluidized bed and subsequent steps continuously without deterioration of the physical properties such as gel forming ability by moistening and drying soybean protein powder particles in a fluidized bed simultaneously or subsequently. In addition, it has been found that it is possible to perform effective coating of a surfactant, which results in further reduction of dustability and further improvement of water dispersibility and that the desired effect can be obtained by using a considerably small amount of a surfactant and therefore taste of the surfactant does not adversely affect the resulting product.